The popularity of mobile data and voice communication continues to grow. The increasing popularity of data and voice communication requires that communication needs of a large number of users must be met, even in situations in which a large number of users are located within a small area. Typical examples for these situations include sports arenas, shopping malls or large office buildings.
The so-called multiple-input and multiple-output (MIMO) technology may be used in wireless radio telecommunication systems for increasing data transmission performance and reliability of a communication between a base station and terminal devices. The terminal devices may comprise for example mobile devices like mobile telephones, mobile computers and tablet computers as well as stationary devices like personal computers or cash registers. Typically, a terminal device may be assigned to a user and may therefore also be called user equipment.
MIMO systems may use multiple send and receive antennas for wireless communication at the base station as well as at the terminal device. The MIMO technology forms the basis for coding techniques which use the temporal as well as the spatial dimension for transmitting information. The enhanced coding in MIMO systems may increase the spectral and energy efficiency of the wireless communication.
The spatial dimension may be used by spatial multiplexing. The spatial multiplexing is a transmission technique in MIMO systems to transmit independent data signals, so-called streams, from each of the multiple antennas. Thus, a plurality of streams may be transmitted at the same time in parallel which is called a higher rank operation. For example, the rank may indicate the number of spatially separated data transmission streams between a user equipment and a base station. For example, a system with a base station having two antennas and the terminal device having two antennas, has the capability of operating in a rank of maximum 2 and is also called 2×2 system, indicating the number of antennas at the base station and at the terminal device.
For establishing the transmission of multiple independent data signals in a MIMO system, antenna parameters of the plurality of antennas have to be configured appropriately. For example phase, gain and amplitude parameters have to be set for each antenna. These parameters may depend on radio channel characteristics for each transmission. Therefore, in a MIMO system channel characteristics between the base station and the terminal device may be analyzed by a channel sounding procedure. The channel sounding procedure may include the transmission of pilot signals at predetermined times having a predetermined coding.
Additionally, or as an alternative, to meet the increasing demands in wireless communication, the radio communication may be moved to higher frequencies, for example radio signals with a frequency of several gigahertz may be used. For example, a frequency of 30 GHz or above may be used. However, when operating at higher frequencies, the aperture of a single antenna element becomes small. The antenna aperture, or effective area, defines how much power the antenna is exposed to. This will decrease transmission efficiency. For mitigating this, multiple antennas may be used for beamforming. However, this may raise the problem how to combine the signals from the multiple antennas for best performance.
In particular, in small terminal devices like mobile telephones or wearable devices, an antenna array comprising a plurality of antenna elements may comprise conformal antennas which may be arranged at different locations of the terminal device and which may have significantly different transmission properties with respect to polarization, phase and gain for different directions. Compared with an antenna array comprising a plurality of similar or identical antenna elements arranged in a regular form like a row (uniform linear array, ULA), a cooperating configuration of conformal antennas may be a complex task to steer. Hence there is no simple formula to create a steering vector. Furthermore, in poor transmission conditions, pilot signals of the channel sounding procedure may be near or below the noise floor making them hard to detect for configuration of the antenna elements.